The invention relates to methods of fabricating low alloy steel ingots and to steel parts that can be obtained by such methods.
When making high integrity mechanical parts that are subjected to high levels of alternating stresses, it can be necessary to design them by using minimum curves covering all of the results characterizing the looked-for properties, including fatigue strength properties. Unfortunately, minimum design curves depend not only on the mean value but also on the dispersion of results. This is particularly true for parts that are used in aviation for which statistical analysis is generally taken into account. Reducing the dispersion of results thus makes it possible to raise the minimum design curves and consequently to improve the performance of parts, e.g. by enabling them to be lighter in weight, to have a longer lifetime, or to increase the stresses to which they may be exposed. Reducing the dispersion of results advantageously makes it possible to obtain a competitive technical difference and economic savings in terms of material used.
Lifetime during oligo-cyclic fatigue stresses may depend firstly on the energy consumed at the moment of initiation on one of the particles present in the metal material that leads to microcracking, and secondly on the propagation of the crack.
Because of a lack of accommodation, certain particles may be subjected to premature cracking, reducing initiation energy, and consequently reducing lifetime compared with the matrix on its own. The nature of the particle, its shape, its individual size, its spatial distribution, and its tendency to cluster with other particles can all have a direct influence on reducing this initialization energy. Dispersion in the types of flaw can lead to wide dispersion in reductions of initiation energy and can consequently correspondingly lower even farther the curve covering the minimum points (by lowering the mean and increasing the standard deviation).
This can apply in particular for steels, and more particularly for remelted low alloy steels. It is known to fabricate grades of steel by remelting metal in an evacuated electric arc furnace (using a vacuum arc remelting method). Such a step serves to improve the inclusion cleanliness by filtering out certain particles that were already present in the metal before such remelting.
With low alloy steels, the presence of included particles of the sulfide and/or oxide type, whether isolated, agglomerated, or in alignment can have an influence on oligo-cyclic fatigue lifetimes. The operations prior to remelting that are presently performed seek to minimize the probability of such particles being present.
Nevertheless, there can remain both particles that are exogenous, and particles that reform during cooling as a result of poor solubility.
In addition, it can be desirable to implement the most stable possible remelting method in order to cause rafts of oxides and sulfides to float in regular manner at the surface of the liquid, going from the center towards the edge of the crucible in the furnace. Nevertheless, each remelting furnace presents a certain amount of dispersion, thereby giving rise to dispersion in the sizes of these flaws, and consequently giving rise to disparities in the lifetimes of the products that are obtained.
There exists a need to be able to obtain low alloy steel parts that present improved lifetimes.
There exists a need to obtain low alloy steel parts that present disparities that are smaller in terms of mechanical properties.
There exists a need to obtain methods of fabricating low alloy steel that make it possible to reduce the impact of instabilities of the remelting furnace.
There also exists a need to have new methods of fabricating parts made of low alloy steel.